1. Technical Field
The invention is related to methods for testing laser target designator systems and in particular to methods for determining both (1) the amount of static misalignment or boresight error between an imaging aim sensor and the laser of the apparatus and (2) the dynamic tracking error of the apparatus.
2. Background Art
Referring to FIG. 1, a laser target designator to be tested (unit under test or UUT) 100 includes an imaging sensor such as a forward looking infrared (FLIR) sensor 102 (and/or a visual sensor) and a laser 104. Any misalignment will cause the laser 104 to illuminate objects not at the center of the field of view of the FLIR 102. The FLIR 102 permits a human operator to place the beam of the laser 104 onto an object by moving the laser target designator or UUT 100 until the desired object is in the center of the field of view of the FLIR 102, typically indicated by cross-hairs in a video display generated by the FLIR 102.
A serious problem with laser target designators is that any misalignment between the optical axes 106, 108 of the FLIR 102 and laser 104, respectively, may cause an object other than that centered by the operator in the FLIR video image cross hairs to be illuminated by the laser 104. Such an error is referred to herein as static error or static boresight error. In those applications in which a "smart" weapon flies to the object illuminated by the laser 104, such an error is unacceptable.
Once the operator locates the FLIR video display cross hairs onto a desired target in the image, he can command a FLIR video processor 110 to have a servo move the FLIR 102 and laser 104 together so as to follow any movement of the target to maintain it in the cross hairs. For this purpose, the FLIR video processor 110 controls a pair of servos 112, 114 controlling rotation of a gimballed platform 116 about horizontal and vertical axes 118, 120, respectively. The FLIR 102 and the laser 104 are mounted on the platform 116 and therefore move with it. The FLIR video processor 110 performs video tracking control of the type well-known in the art, using conventional video processing and feedback control techniques to track a target in the image so that the laser 104 continues to illuminate the target as long as the operator desires even while the target is moving.
One problem with such a video tracking system is that there are certain inherent inaccuracies and delays arising from two sources of error. One error source is the electromechanical limitations of the servos 112, 114 and the gimbal mechanics associated therewith. Another error source is the electronic limitations of the FLIR video processor 110 and the limited image resolution of the video image with which the processor 110 must work with. Yet another error source is the alignment error between the laser and its aim reticles. Together, these error sources give rise to significant delays and inaccuracies of the video tracking system. As a result, the laser beam does not accurately follow a moving target and there is therefore some risk that a quickly moving target can evade the laser guided weapon. This latter servo error is referred to herein as dynamic error.
Another major problem with such tracking systems is that laser target designators must be tested prior to actual use in order to verify that the static boresight error is within acceptable limits. The FLIR 102 operates in the 8-12 micron wavelength region while the laser 104 typically operates in the 1.06 micron wavelength region. Automatic measurement of misalignment between the FLIR and laser optical axes 106, 108 typically has required either expensive multispectral beam splitters or movement of optical elements to switch between (1) a thermal source which stimulates the FLIR 102 at infrared wavelengths and (2) an optical sensor which senses the beam from the laser 104 at optical wavelengths. These elements introduce large errors due to vibration and time-dependent thermal drift.
Some testing techniques try to improve accuracy by introducing a glass target illuminated by the laser 104, the FLIR 102 sensing the hot spot thus produced in the glass target. This produces an image which the operator can check for misalignment of the laser beam relative to the center of the field of view of the FLIR 102. The problem with such an approach is that the hot spot can move due to vibration, and it diffuses over time, making the misalignment measurement unreliable. Also, such a method cannot measure dynamic error.
One limitation of the testing technique illustrated in FIG. 1 is that the displacement between the optical paths of the laser 104 and the FLIR 102 requires a long range to the field target board for accurate results, a significant disadvantage.
Due to the foregoing problems, measurements of static error in a laser target designator have been accurate to on the order of only a few milliradians, whereas it is necessary to be able to measure such errors to within fractions (e.g., hundredths) of one milliradian. Moreover, the need to measure the dynamic error of a laser target designator in the laboratory or portable shelter has not been substantively addressed in the art.